High Spatial Resolution Optical and Radio Imagery of the Circumbinary Environment

0.55 MB·English

by  M. F. Bode

Symbiotic stars probing stellar evolution
ASP Conference Series, Vol. xxx, 2002
R. L. M. Corradi, J. Mikolajewska, and T. J. Mahoney, eds.
High Spatial Resolution Optical and Radio Imagery of the
Circumbinary Environment
Michael Bode
Astrophysics Research Institute, Liverpool John Moores University,
4 Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, United
0 Kingdom
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Abstract. In this review, I concentrate on describing observations of
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spatially resolved emission in symbiotic stars at sub-arcsecond scales. In
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J some of the closer objects, the highest resolutions discussed here corre-

4 spond to linear dimensions similar to the supposed binary separation. A
2 total of 17 stars well accepted as symbiotics are now observed to show

sub-arcsecond structure, almost twice the number at the time of the last
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review in 1987. Furthermore, we now have access to HST imagery to
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9 add to radio interferometry. From such observations we can derive fun-
6 damental parameters of the central systems, investigate the variation of
4 physical parameters across the resolved nebulae and probe the physical
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mechanisms of mass loss and interactions between ejecta and the circum-
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stellarmedium. Suggestionsforfutureworkaremadeandthepotentialof
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0 new facilities in both the radio and optical domains is described. This re-
/ view complements that by Corradi (this volume) which mainly considers
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p the larger scale emission from the ionized nebulae of these objects.
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: 1. Introduction
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It is now fifteen years since Taylor’s review of high resolution radio imagery
r of symbiotic stars was presented at the Torun conference (Taylor 1988). In
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that time, we have not only seen the sensitivity and frequency range of radio
interferometers such as the VLA and MERLIN increase, but also, and perhaps
more importantly, we have witnessed the advent of the Hubble Space Telescope.
The HST has allowed “routine” sub-arcsecond imaging in optical wavebands.
Taking the capabilities of each of these facilities in turn:
The Very Large Array. The VLA can operate in 8 frequency bands for
continuum work from 0.07 to 43 GHz. In its largest configuration (A-array)
with a baseline of 36 km, it can achieve 0.04 arcsecond resolution. At 5 GHz,
its rms sensitivity is quoted as 50 µJy/beam in 10 minutes.
MERLIN. Possibly less well known outside the international radio com-
munity are the capabilities of the MERLIN array, centered on Jodrell Bank.
MERLIN comprises an array of radio telescopes distributed across the UK with
a maximum baseline of 217 km. It operates in three main bands at 1.7, 5 and
22 GHz. At the highest of these frequencies, MERLIN has a resolution of 0.008
arcseconds. It can achieve an rms sensitivity of 50 µJy/beam in a 12 hourtrack.
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2 Author & Co-author
HST. The primary instruments that have provided the results discussed in
this review are the Wide Field and Planetary Camera (WFPC2) and the Space
Telescope Imaging Spectrograph(STIS).Both instrumentscan performimaging
througha rangeof filters. STIScan of coursealso performlong slitspectroscopy
at a range of spectral resolutions. The operating wavelength range of these
instruments is approximately 1200 to 11,000 ˚A and the spatial resolution of
order 0.1 arcseconds.
From this, it can be seen that for a “typical” symbiotic at a distance of
1kpc, MERLIN for example can theoretically resolve features with spatial ex-
tent ∼ 8 AU, which is only a few times larger than the binary separation in
S-types (and of course, D-types have much larger separations). With observa-
tions down to these scales, in combination with modeling, and particularly if
temporal variations can bemonitored, we could potentially answer thefollowing
fundamental questions:
• What are the orbital parameters (particularly of D-types)?
• Howisthenebularmorphologylinkedtotheparametersandactivity ofthe
central system, and can we test competing models of nebular formation?
• Are we really seeing jets in some sources, and if so, what is their origin?
• Howiscircumstellardustdistributedandcanwedisentanglethetruevalue
of interstellar extinction?
• Are there additional systematic differences between different sub-types?
It turns out that in some instances we can now make considerable headway
on several of these points. However, as shown in the final section, the real
breakthroughs will come with the next generation of ground-based facilities now
in advanced planning or the early phases of construction.
2. Review of Individual Objects
Taylor (1988) discussed the limitations of current radio interferometers for de-
tecting spatially extended emission in symbiotic stars. He then listed 10 objects
where structurehad been resolved at sub-arcsecond size scales using radio inter-
ferometry. In Table 1 below are listed all the objects generally accepted to lie
within the symbiotic classification where such structure has been detected and
the results subsequently presented in published sources. The list was compiled
up to May 2002 and objects are shown in the order they are discussed in the
text. The number of sources has thus expanded to 17 and includes HST and
AustraliaTelescope(AT)detections. TheseofcoursedidnotfigureintheTaylor
review. In the case of therecurrentnova RSOph, often includedin the category
of symbiotic, we were able to secure European VLBI Network observations dur-
ing its 1985 outburst which illustrate what the highest spatial resolution radio
observations can reveal (Taylor et al. 1989). Similarly, the Cambridge Optical
Aperture Synthesis Telescope (COAST, see e.g. Baldwin & Haniff 2001) is pi-
oneering the use of optical interferometry and is discussed further in the final
section.
APS Conf. Ser. Style 3
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Table 1. Spatially Resolved Emission at ∼< 1 arcsec
Object Sub-type Detected Using Principal References
R Aqr D VLA, MERLIN, HST 1,2,3
RX Pup D VLA, HST 4,5,6
HM Sge D VLA, MERLIN, HST 7,8,9,10,11
V1016 Cyg D VLA, MERLIN, HST 12,13
H1-36 D VLA 14
He2-104 D HST 15
BI Cru D AT 7
He2-106 D AT 7
RR Tel D AT 7
HD149427 D′ AT, HST 7,16
Z And S VLA, MERLIN 7
AG Peg S VLA, MERLIN 7,17
AG Dra S MERLIN 18
CH Cyg S VLA, MERLIN, HST, COAST 19,20,21,22,23
SS73 96 S VLA 14
Hen 3-1383 ? VLA 14
RS Oph RN VLA, EVN 24,25
We now proceed to discuss individual objects of particular interest in more
detail.
2.1. D-types
R Aquarii The large scale nebula surrounding R Aqr has been the subject of
a great deal of detailed work over several decades (see Gonc¸alves, this volume).
High resolution radio and optical (HST) observations of the inner nebula have
been performed by several groups of recent years.
Forexample,Doughertyetal. (1995)observedRAqrat1.7and5 GHzwith
MERLINyieldingresolutionsof130and40 milliarcseconds(m.a.s.) respectively.
InFig. 1areshownthe5 GHzresults. Doughertyetal. arguethatthesymbiotic
binary pair is located within what they designate as feature C1c. Spectral index
mapping made possible by the quasi-simultaneous securing of observations at
the two frequencies shows that thepeak spectralindex (α = 1.6) is located here.
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Assuming a spherically symmetric isothermal wind (T = 10 K) of constant
outflow velocity (v = 30 km s−1) emitting via the free-free process (i.e. a
Wright & Barlow 1975 model) gives M˙ = 1.3×10−8 M⊙ yr−1 which is very low
for a Mira variable. However, applying an ionization-bounded (X ∼ 0.04)
STB
1References: 1. Dougherty et al. 1995; 2. Hollis et al. 1997; 3. Hollis et al. 2001; 4. Hollis
et al. 1989; 5. Corradi & Schwarz 2000; 6. Paresce 1990; 7. Kenny 1995; 8. Solf 1984; 9.
Eyres et al. 1995; 10. Richards et al. 1999; 11. Eyres et al. 2001; 12. Watson et al. 2000;
13. Brocksopp et al. 2002; 14. Taylor 1988; 15. Corradi et al. 2001a; 16. Parthasarathy et al.
2000; 17. Kennyetal. 1991; 18. Mikolajewska 2002; 19. Taylor etal. 1986; 20. Crockeret al.
2001; 21. Eyreset al. 2002; 22. Corradi et al., 2001; 23. Crocker et al. 2002; 24. Hjellming et
al. 1986; 25. Taylor et al. 1989
4 Author & Co-author
Figure 1. 5 GHz MERLIN observations of R Aqr (Dougherty et al.
1995, contours)superimposedonHST190 nmimageofParesce&Hack
(1994, grayscale). Positions of features identified by previous authors
are indicated, including those of the Long Period Variable and maser
sources. The central binary is located in the most westerly radio peak
(“feature C1c” - see Dougherty et al. for full details).
STB (Seaquist, Taylor & Button, 1984) model with v = 10−30 km s−1 gives a
much more reasonable M˙ =5−15×10−7 M⊙ yr−1.
Hollis,Pedelty&Lyon(1997)havesubsequentlyusedtheVLAat43 GHzto
mapfeatureC1catthefrequencyoftheSiOmaser(revealingapointsource)and
the adjacent continuum. Thelatter shows bipolar structure. If this is associated
with emission pertaining to the binary components, and the separation of the
peaks is the projected separation of the binary, then for M ∼ 2.5−3M⊙ and
P ∼ 44 years, a ∼ 17−18AU. Using preliminary orbital elements, Hollis et al.
determine a distance to R Aqr of 195-206pc, very much in line with estimates
from other methods. They also suggested that a monitoring campaign with the
VLA at this frequency would obviously yield more precise system parameters.
Most recently, Hollis et al. (2001) have used high spectral and spatial resolution
observationswiththeVLBAintheν = 1,J = 1−0SiOmaserline. Theirresults
are consistent with Keplerian (differential) rotation of the SiO maser shell.
RX Puppis RX Pup is another symbiotic Mira system. Hollis et al. (1989)
observedRXPupat5and15 GHzwiththeVLAin1986. Theyfoundapossible
northern elongation in the 5 GHz data and (more convincingly perhaps) found
three components aligned approximately E-W at the higher frequency on sub-
arcsecond scales. The brightest of these is associated by these authors with the
central star. They speculate that the other two features arise from a jet and
that the morphology is reminiscent of that of R Aqr. Corradi & Schwarz (2000)
discuss optical spectroscopy of the region of an elongation at p.a. 15◦ extending
to ∼ 3.7 arcsec from the central star and firstrevealed in coronographic imaging
APS Conf. Ser. Style 5
byParesce(1990). Theyconfirmthepresenceofthefeature,butfindnoevidence
of this also being a jet. A tentative detection of elongated emission E-W was
also made which may be related to the extended emission seen at 15 GHz by
Hollis et al.
HM Sagittae HM Sge is an outbursting symbiotic that underwent a major
brightening in 1975. It has been observed in the radio by several workers at
many epochs since then. VLA observations have shown extended structure on
size scales from arcminutes to sub-arcsecond (e.g. Kenny 1995). As pointed
out by Taylor (1988), the bipolar structure seen at the highest resolutions is
very similar to that observed in V1016 Cyg. Indeed these two outbursting D-
types sharemany similar characteristics (see below). Observations of HMSgeat
22 GHz by Li (1993) suggested “rotation” of the inner components (separation
∼ 0.1 arcsec) implying a period of 65-118 years.
In the optical, Solf (1984) performedheroic ground-basedspatially-resolved
spectroscopic observations. From these he deduced the existence of a bipolar
mass flow in two lobes with separation 1.5 arcseconds (roughly E-W), two low
velocity features lying N-S and separated by 0.2 arcseconds (deduced to be
either a rotating ring or slowly expanding blobs) and a shell of emission around
0.5 arcseconds in diameter expanding at an intermediate velocity of ≃ 60 km
s−1. Solf then used these data to deducea distance to HM Sge of approximately
400 pc.
Inhisthesis,Kenny(1995)providesradioobservationsofseveralsymbiotics,
including HM Sge. He also develops models of their circumstellar environments,
building on work of e.g. Kwok, Purton & Fitzgerald (1978, concentric colliding
winds - CWc), Girard & Willson (1987, binary colliding winds - CWb) and
Seaquist et al. (1984, STB). Figure 2 shows a composite colliding wind model
coupled with an STB model, plus rotation. In the radio, such a confiiguration
will naturally lead to peaks in the observed thermal emission which will rotate
with the binary period. Projection effects due to orbital inclination may then
lead to changes in the apparent angular separation of the features.
Monitoring of HM Sge (and other symbiotics) has continued using both the
VLA and MERLIN (Eyres et al. 1995; Richards et al. 1999), and most recently
in combination with HST WFPC2 imaging (Eyres et al 2001). In Figure 3,
Northern(N)andSouthern(S)componentsofradioemissionseparatedat5 GHz
by ∼ 150 m.a.s. are shown. Richards et al. again found an apparent rotation of
radio components which is in line with the Kenny models and suggests a binary
periodP = 90±20 years. Furthermore, combination of MERLIN1.7 and5 GHz
observations allowed spectral index maps to be derived. Figure 4 clearly shows
that although the emission near the supposed stellar position is dominated by
optically thick thermal bremsstrahlung, as one moves E or W of the central
source, the spectral index declines until non-thermal (synchrotron) emission is
obviously dominant.
Aside from supernova remnants, spatially resolved, non-thermal emission
is very rare in stellar sources and of course is the signature of particle acceler-
ation and magnetic field enhancement in the circumstellar environment. Such
effects would beexpected in wind-ejecta interaction regions, andthe anticipated
presence of enhanced magnetic fields in the environs of the cool component is
discussed in the review of Soker (this volume). From a simple model, it is con-
6 Author & Co-author
Figure 2. Hybrid STB/CW model for HM Sge. Hot component de-
noted by ◦, cool component by •. Orbital motion of the stars (bottom
panel)leadstochangingp.a. betweenthenebularfeatures(fromKenny
1995 - see text for further details).
cluded that this emission should decline within decades, a prediction which may
helptoexplainthenon-detectionofsuchemissionnowinV1016Cyg(seebelow).
InHSTGOprogram8330, Eyresetal. (2001)usedselectedWFPC2narrow
bandandbroadbandfilterstoimagethenebularemission(a)atthewavelengths
of diagnostic lines and (b) to locate the hot and cool continuum sources respec-
tively. These observations were conducted on 1999 October 22. HM Sge had
also been observed with the VLA in A-array at 8.56 and 23 GHz as part of the
samestudyon1999September26. Results areshowninFig. 5. AswellastheN
and S features, a central peak (C) is indicated. This is where the central binary
almost certainly lies. Several other larger scale nebular features are indicated.
The [OIII] line ratio map clearly shows a “wedge” of cooler material to the SW,
extending ∼ 0.5 arcseconds from the central regions of the nebula. Although
it is dangerous to deduce precise temperatures from such ratios, the qualitative
effects of temperature variation are certainly indicated.
Acombination of Hβ andradio continuum images have allowed theproduc-
tion of an extinction map of the innermost regions of the nebula. The principal
results are that the peak of extinction lies in the direction of the cool wedge
to the South of the central emission with a value EB−V ∼ 1. The minimum
APS Conf. Ser. Style 7
Figure 3. MERLIN 5 GHz observations of HM Sge at four epochs
(Richards et al. 1999). The mean optical position is also marked.
value of extinction across the map is found to be EB−V ∼ 0.35. If this truly
is the minimum value, then it is a measure of the interstellar extinction to HM
Sge and by comparison with large scale interstellar extinction maps, this places
the object at > 700pc. (NB: The λ2200 feature is not a good indicator of total
extinction toadustysourceasitis onlystrongly correlated withtheinterstellar,
and not the circumstellar component of extinction - see e.g. Bode 1987).
The broad band filters were chosen specifically to include only spectral re-
gions expected to be dominated by emission from the hot and cool continua
associated with the stellar components themselves. Using 2D gaussian fitting
to dithered images, centroids of emission were calculated. These showed a po-
sitional offset of 40±9 m.a.s. at a p.a. of 130◦ ±10◦ with the cool component
being the more southerly. It should be noted that polarimetic work of Schmid
et al. (2000, see also the more general review in this volume) gave a consistent
p.a. of 123◦ for the line joining the stellar components. Taking a = 25 AU
(consistent with P = 90 years; M = 2 M⊙), and the fact that from the radio
observations we expect the binary to have been observed at around the time of
greatest elongation, gives d = 625±140 pc. This is on the low side of previous
estimates of the distance to HM Sge. Accurate positions of emission peaks were
also derived through the narrow band (line) filters. For example, this placed
8 Author & Co-author
Figure 4. HM Sge spectral index between 5 and 1.6 GHz at two
epochsonanE-Wslice. Verticallinesindicateapproximateboundaries
of the E and W components of non-thermal emission first detected by
Eyres et al. (1995). Dotted and dashed lines indicate positions of
optical and radio central features respectively (from Richards et al.
1999).
the peak of the HeII emission between the stellar components, consistent with
the model of Nussbaumer & Vogel (1990). Results consistent with expectations
are also found for V1016 Cyg and CH Cyg (see below). However, because of
the potential importance of these results, investigations are still continuing to
determine whether the offsets in positions could be due to a subtle systematic
effect. Unfortunately, at the present time it is proving difficult to arrive at a
definitive answer (see also Brocksopp et al., this volume).
V1016 Cyg As noted above, this object is the virtual twin of HM Sge in many
ways. V1016 Cyg also underwent a nova-like outburst (in 1965), it shows a
bipolar radio morphology on sub-arcsecond scales (see e.g. Taylor 1988) and
Solf (1983) had deduced a very similar optical morphology to HM Sge. Recent
MERLIN 5 GHz observations over a period of several years have indicated that
as with HM Sge, there is an apparent rotation of the central features (Watson
et al. 2000 - see also Fig. 6). Comparison of Figs. 3 and 6 emphasizes how
remarkably similar the radio morphologies of these two objects are at present.
Unlike HM Sge however, spectral index mapping did not reveal any extended
non-thermal component to the emission.
V1016 Cyg was also a target of HST GO program 8330. Again, a range of
filters was used to map emission at various diagnostic line wavelengths and to
locate the positions of the binary components. VLA observations at 23 GHz in
APS Conf. Ser. Style 9
Figure 5. HST images of HM Sge in three WFPC2 filters (a-c) with
main resolved features indicated. (d) shows the ratio map of emission
in (a) and (b) (Eyres et al. 2001).
A-array were made within 6 weeks of those with HST. These data reveal other
similarities with HM Sge in terms of apparent binary separation and position
angle on the sky. In addition, the sub-arcsecond nebula was observed with STIS
as part of a spectroscopic SNAP program (8188) with HST. The differences
between the two objects (e.g. the greater observable structure in HM Sge; the
presence or otherwise of extended non-thermal emission) can be explained in
terms of the probability that V1016 Cyg is more distant, and certainly had an
earlier outburst. A full discussion of the results of all these recent radio and
HST observations is given in Brocksopp et al. (2002 - see also contribution in
this volume).
2.2. S-types
Z Andromedae Z And is an active system that has undergone several recorded
outbursts (see e.g. Fernandez-Castro et al. 1995). Kenny (1995) discusses
near-simultaneous MERLIN and VLA observations in 1991-1992. These reveal
a central source extended by ∼ 0.1 arcsec E-W. Kenny fitted these observations
with a simple STBmodel plus a “no recombination” radius of 70 AU (this being
the radius from the central system at which ionized gas does not have time to
recombine before the ionization cone around the hot component sweeps through
again as the binary rotates). The best fit model (both to the images and radio
10 Author & Co-author
Figure 6. MERLIN maps at 5 GHz of V1016 Cyg from 1992 July
21 (top left), 1995 June 25 (bottom left) and 1996 November 9/1997
February 3 (top right). Also shown, with main resolved features indi-
cated, is a composite image of all four epochs (Watson et al. 2000).
light curves) gave M˙ /v = 9.5×10−9 M⊙ yr−1(km s−1)−1, Lph = 2.6×1045 s−1,
X = 0.36 and θ = 30◦ for a = 2 AU and d = 1.1 kpc (where L is the
STB a ph
hot component ionizing photon flux, v is the red giant wind velocity and θ is
a
the opening angle of the cone of ionized material). In addition to the central
extended source, a “blob” of emission ∼ 0.1 arcsec to the North was suggested
as possibly being collimated ejecta from the 1984/85 outburst of the system.
AG Pegasi AG Peg is an archetypal symbiotic nova that underwent a ma-
jor outburst in ca.1850. It shows structure in the radio from arcminute to
sub-arcsecond scales (Kenny, Taylor & Seaquist 1991). Kenny (1995) reports
MERLIN plus VLA observations. He has modeled the emission via colliding
winds with vhot = 900 km s−1, vcool = 10 km s−1, M˙cool = 2.1×10−7 M⊙yr−1,
i = 65◦ and projected polar axis p.a. = −15◦ for a = 2.5 AU, d = 600 pc (see
Fig.7). Variation in the resolved images over the three year period is deduced
to be due to orbital motion and a factor ∼3 decrease in mass loss rate from the
hot component to 0.1M˙ in 1990.
cool
AG Draconis Mikolajewska (2002, also this volume) has drawn attention to
the fact that the MERLIN images of AG Dra in Ogley et al. (2002) show two